1. Field of the Invention
The present invention is directed to a high frequency switching device, and more particularly to a miniature switching device such as a relay for switching high frequency signals.
2. Description of the Prior Art
A known high frequency switching device has a contact block having fixed contacts and a movable contact which is driven to open and close a current path between the fixed contacts. In order to isolate the current path from a surrounding electromagnetic field, the fixed contacts and the movable contact are surrounded by an electromagnetic shield which is supported on a conductor base and is grounded therethrough. The fixed contacts are formed respectively on one ends of terminal pins provided for electrical connection to an external load circuit operating on high frequency signals. The terminal pin extends through an insulation ring fitted in the conductor base so as to be electrically insulated therefrom. Retaining this structure for high frequency use, the device has been demanded to be made compact enough to be incorporated into small electronic devices, such as portable cellular phones, while retaining minimum return loss, i.e., having VSWR as nearly to unity as possible. In order to achieve miniaturization of the device while keeping an optimum over-all impedance of about 50.OMEGA. along a signal path of the high frequency current, however, it should be taken into account that a short segment of the terminal pin passing through the insulation ring is restricted to have a specific impedance (Z) expressed in the following equation. ##EQU1## where .epsilon.r is a dielectric constant of the insulation ring.
Therefore, provided that the terminal pin is uniform in diameter along its length, the signal path along the terminal pin suffers from varying impedance at this particular segment surrounded by the insulation ring. In fact, the impedance at this segment will be considerably lowered with respect to the other portion of the signal path. Although it is possible to adjust the impedance (Z) at this segment to the optimum impedance of about 50 .OMEGA. by suitably selecting dielectric constant .epsilon.r, thickness (d) of the base and diameter (D) of the insulation ring, all the attempts are found not practically acceptable for the following reasons. Firstly, the dielectric constant .epsilon.r is substantially fixed because of limited availability of the dielectric material as the insulation ring. Secondly, the thickness (d) of the conductor base should not be made so small in order to retain mechanical strength. Thirdly, the diameter (D) should not be made so large as to be consistent with the miniaturization of the device. Consequently, no effective scheme has been found to combine the miniaturization of the relay and the over-all optimum impedance of the signal path.